Notice : This addon won't work on firefox versions > 56. The new extension sdk (WebExtensions) does not provide the API needed. Sorry.
In newer Firefox versions, "open in tabs" doesn't replace the tabs by the new ones, like Firefox2 did. With this add-on, this option is restored. There no configuration for this add-on, desactivate it to disable it.
This add-on is for all the users who want the tabs to be closed before "opening in tabs" a directory containing bookmarks.
Note that this option is enabled only when "opening in tabs" with middle click. If you want to keep the current tabs opened, you can still "open in tabs" new bookmarks with the right click menu.